"This number of seismicity is an unprecedented seismic observation at Agung volcano ever recorded by our seismic networks," it said in a statement.

Earlier, the Department of Meteorology, Climate and Geophysics said in a statement there has been a "tremendous increase" in seismic activity at the mountain, indicating a greater probability of an eruption.

Camera IconVillagers rest after being evacuated during the raised alert levels for the volcano on Mount Agung in Klungkung regency on the Indonesian resort island of Bali. Credit: AFP

Indonesia’s National Disaster Mitigation Agency spokesman Sutopo Purwo Nugroho said overnight that the hazardous zone had been increased from 9km to 12km, covering an area encompassing about 240,000 people and prompting further evacuations. He urged people to "calm down" and seek reliable information.

"Estimated danger zones are dynamic and are being continuously evaluated. (They) are subject to change at any time following the most recent observation data," Indonesia’s volcano observation authority warned.

"In case of eruption, the potential primary hazard that may occur within a radius of 9km is pyroclastic fall of size equal to or greater than 6cm," its website on Agung reads.

But its modelling for some of the terrain around the volcano also shows such flows could cover 10km in less than 3 minutes.

"If an eruption occurs, there is considerable disaster potential," it warns. "People around Mount Agung also do not have enough experience to face the eruption because this volcano last erupted ... 54 years ago."

Camera IconIndonesian authorities have raised the alert level for the Mount Agung volcano on the tourist island of Bali to the highest level. Credit: AP Photo/Firdia Lisnawati

Agung last erupted in 1963, unleashing deadly pyroclastic flows which killed about 1100 people and hurling ash as high as 10 kilometres.

It is just one of 130 active volcanoes in Indonesia, part of the Pacific "Ring of Fire" convergence of tectonic plates.

The National Disaster Mitigation Agency earlier said no residents or tourists should be within nine kilometres of the crater and within 12 kilometres to the north, north-east, south-east and south-south-west. The volcano is about 70km from the tourist hub of Kuta.

The agency earlier said about 9400 villagers have evacuated from their houses and are scattered across the districts of Karangasem, Klungkung and Buleleng.

Camera IconVillagers who were evacuated from their homes on the slope of Mount Agung sit outside tents prepared to become their temporary shelter in Klungkung, Bali. Credit: AP

They are living in temporary shelters, sports centres, village halls and with relatives, said agency spokesman Sutopo Purwo Nugroho.

More than 7000 of the evacuees were in the 840 square kilometre district surrounding the volcano, Karangasem, which is home to about 408,000 people.

The Australian Department of Foreign Affairs issued an updated travel warning for Indonesia that said an eruption could impact air travel in the region.